Instrumentation amplifier with rail-to-rail input range

ABSTRACT

A system and method for adjusting a common mode output voltage in an instrumentation amplifier is provided. In one aspect, the common mode output voltage is increased or decreased with respect to the common mode input voltage to enable high amplification of the signal input to the instrumentation amplifier. Moreover, the common mode output voltage can be driven to (or approximately to) a target voltage value such as, but not limited to, half the supply, even if the common mode input voltage is close to supply or ground rail voltage. Thus, a high amplification of the differential input voltage can be obtained and utilized for various applications requiring rail to rail input.

TECHNICAL FIELD

The subject disclosure relates to instrumentation amplifiers and, moreparticularly, to an instrumentation amplifier with a rail-to-rail inputrange.

BACKGROUND

Instrumentation amplifiers are extensively used in analog systems toamplify low level signals in the presence of high common mode noise. Asan example, instrumentation amplifiers are typically employed as inputamplifiers of signal processing systems. Moreover, instrumentationamplifiers can amplify a differential input signal with a high rejectionof common mode input voltage. Typically, instrumentation amplifiers havehigh input impedance, low input current, and good linearity.

The common mode input range of the amplifier is limited by the followingconstraint: The input stage of the two amplifiers constituting theinstrumentation amplifier must be properly biased. To overcome thisconstraint, conventional systems employ two differential input pairs oftransistors within the instrumentation amplifier, one based on n-channelmetal-oxide-semiconductor (NMOS) transistors, and one based on p-channelmetal-oxide-semiconductor (PMOS) transistors. The PMOS baseddifferential input transistor pair works accurately for a low commonmode input voltage range but not for a common mode input voltage rangethat is close to the positive supply voltage (Vdd). On the other hand,the NMOS based differential transistor pair works accurately for a highcommon mode input voltage range, but not when the common mode inputvoltage range is close to the negative supply rail voltage (Vss). Thus,by combining PMOS and NMOS based differential transistor pairs, theinput stages of the conventional amplifiers constituting thedifferential amplifier can have a rail to rail input range. However, thegain and hence the performance of the conventional amplifier falls downrapidly when the output voltage approaches the positive or negativesupply rail (e.g., within a few tens of millivolts).

SUMMARY

The following presents a simplified summary of the specification inorder to provide a basic understanding of some aspects of thespecification. This summary is not an extensive overview of thespecification. It is intended to neither identify key or criticalelements of the specification nor delineate any scope particularembodiments of the specification, or any scope of the claims. Its solepurpose is to present some concepts of the specification in a simplifiedform as a prelude to the more detailed description that is presentedlater. It will also be appreciated that the detailed description mayinclude additional or alternative embodiments beyond those described inthis summary.

The systems and methods disclosed herein, in one aspect thereof, canfacilitate increasing an input range of an instrumentation amplifier. Inone aspect, the subject system can include an instrumentation amplifierthat amplifies a differential input voltage signal to generate adifferential output voltage signal; and a feedback circuit that controlsa common mode output voltage of the instrumentation amplifier based onthe differential output voltage signal. Another aspect relates to asystem comprising a first circuit that is configured to determine acommon mode output voltage of an instrumentation amplifier based on theoutput voltage signals of the instrumentation amplifier; and a secondcircuit that is configured to generate a control signal to minimize adifference between the common mode output voltage and a target voltage.Yet another aspect relates to a method comprising measuring the outputvoltage signals from an instrumentation amplifier; and based onanalyzing the output voltage signals, generating a control signal foradjusting the common mode output voltage to prevent saturation of theinstrumentation amplifier.

The following description and the annexed drawings set forth certainillustrative aspects of the specification. These aspects are indicative,however, of but a few of the various ways in which the principles of thespecification may be employed. Other advantages and novel features ofthe specification will become apparent from the following detaileddescription of the specification when considered in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous aspects, embodiments, objects and advantages of the presentinvention will be apparent upon consideration of the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like reference characters refer to like parts throughout, and inwhich:

FIG. 1 illustrates an example system that that facilitates increasing aninput range of an instrumentation amplifier circuit;

FIGS. 2A-2B illustrate example systems for adjusting a common modeoutput voltage of the instrumentation amplifier circuit;

FIG. 3 illustrates an example circuit utilized for controlling thecommon mode output voltage of the instrumentation amplifier circuit byemploying two controlled current sources;

FIG. 4 illustrates an example circuit that facilitates common modecurrent control by employing a controlled current source and provides ahigh common mode input rejection;

FIG. 5 illustrates an example circuit that facilitates controlling avoltage at a node between input resistors of an instrumentationamplifier circuit to adjust the common mode output voltage;

FIG. 6 illustrates an example voltage control circuit utilized tocontrol the common mode output voltage;

FIGS. 7A-7B illustrate example circuits that prevent output voltages ofthe instrumentation amplifier from increasing and/or decreasing outsidethe supply voltage range;

FIGS. 8A-8B illustrate example circuits that determine whether theoutput voltages of the instrumentation amplifier circuit are within apredefined voltage range;

FIG. 9 illustrates an example graph that depicts output voltage of aninstrumentation amplifier with a rail to rail input range;

FIG. 10 illustrates an example method for increasing an input range ofan instrumentation amplifier; and

FIG. 11 illustrates an example method for controlling common mode outputvoltage of the instrumentation amplifier.

DETAILED DESCRIPTION OF THE INVENTION

Systems and methods disclosed herein, in one or more aspects, provide aninstrumentation amplifier with a rail to rail input range. The subjectmatter is described with reference to the drawings, wherein likereference numerals are used to refer to like elements throughout. In thefollowing description, for purposes of explanation, numerous specificdetails are set forth in order to provide a thorough understanding ofthe subject innovation. However, that the subject matter may bepracticed without these specific details.

As used in this application, the term “or” is intended to mean aninclusive “or” rather than an exclusive “or”. That is, unless specifiedotherwise, or clear from context, “X employs A or B” is intended to meanany of the natural inclusive permutations. That is, if X employs A; Xemploys B; or X employs both A and B, then “X employs A or B” issatisfied under any of the foregoing instances. In addition, thearticles “a” and “an” as used in this application and the appendedclaims should generally be construed to mean “one or more” unlessspecified otherwise or clear from context to be directed to a singularform. In addition, the word “coupled” is used herein to mean direct orindirect electrical or mechanical coupling.

Initially, referring to FIG. 1, there illustrated is an example system100 that facilitates increasing an input range of an instrumentationamplifier circuit 102, according to an aspect of the subject disclosure.In one aspect, the instrumentation amplifier circuit 102 can be employedto amplify a relatively small differential input signal, for example,received from a sensor (e.g., resistive of capacitive bridge) whileproviding a high input impedance. Such amplification can be utilized toadapt the range of the input signal to the range of an analog to digital(A/D) converter (not shown) in order to digitize the signal from thesensor.

In one aspect, the instrumentation amplifier circuit 102 can include aset of operational amplifiers (OPAMPs), for example, a differentialamplifier with input buffers that can eliminate the need for inputimpedance matching. As an example, the instrumentation amplifier circuit102 can have a low direct current (DC) offset, low drift, low noise,high open-loop gain, high common-mode rejection ratio, and high inputimpedances. Based on a differential input signal 104, theinstrumentation amplifier circuit 102 can generate an amplifieddifferential signal 106 with a high rejection of common mode inputvoltage. Typically, the common mode input range of the instrumentationamplifier circuit 102 is limited by the following two constraints: (i)the input stage of the two amplifiers constituting the instrumentationamplifier circuit 102 must be properly biased; and (ii) the output nodescannot be driven outside the supply range. Moreover, the gain and hencethe performance of the instrumentation amplifier circuit 102 decreasesrapidly when the output voltage approaches the positive supply rail(Vdd) or the negative supply rail (Vss) (e.g., within a few tens ofmillivolts). In one example, the instrumentation amplifier circuit 102,not only amplifies the differential signal but also transfers the commonmode voltage from input to output with unity gain. Accordingly, a highdifferential gain can only be achieved when the input range is far fromeach supply rails and the output voltages nodes are not clamped by theclosest supply voltage (Vdd or Vss). Thus, to avoid voltage clamping andincrease input range of the instrumentation amplifier circuit 102,system 100 includes a common mode adjustment component 108 thatfacilitates shifting the common mode output voltage with respect to thecommon mode input voltage, in order to allow a high amplification of theinput signal, independent of the common mode input voltage.

In one example, the common mode adjustment component 108 can introduce acontrolled common mode current through feedback resistors within theinstrumentation amplifier circuit 102 and dynamically shift (e.g.,increase or decrease) the common mode output voltage with respect to thecommon mode input voltage. In another example, the common modeadjustment component 108 can control voltage at a control node betweeninput resistors within the instrumentation amplifier circuit 102 anddynamically shift the common mode output voltage with respect to thecommon mode input voltage. Moreover, the common mode adjustmentcomponent 108 can employ most any circuit to drive the common modeoutput voltage close to half of the supply voltage (Vdd/2), for example,even if the common mode input voltage is close to supply or ground rail.Accordingly, the common mode adjustment component 108 can allow theinstrumentation amplifier circuit 102 to realize a high amplificationindependent of the common mode input voltage.

As an example, system 100 can be utilized for applications requiringrail to rail input and/or for applications wherein the common mode inputvoltage is unknown, changes drastically, and/or cannot be estimated inadvance. It can be appreciated that the design of system 100 can includedifferent component selections, topologies, etc., to achieve a controlsignal that dynamically adjusts the common mode output voltage withrespect to the common mode input voltage to prevent voltage clampingand/or errors in the output of the instrumentation amplifier circuit102. Moreover, it can be appreciated that the instrumentation amplifiercircuit 102 and the common mode adjustment component 108 can includemost any electrical circuit(s) that can include components and circuitryelements of any suitable value in order to implement the embodiments ofthe subject innovation. Furthermore, it can be appreciated that thecomponents of system 100 can be implemented on one or more integratedcircuit (IC) chips.

Referring now to FIGS. 2A-2B, there illustrated are example systems(200, 250) for adjusting the common mode output voltage of aninstrumentation amplifier circuit 102, according to an aspect of thespecification. In one aspect, system 200 includes a current controlcircuit 202 that facilitates adjustment of the common mode outputvoltage (e.g., to avoid saturation of the instrumentation amplifiercircuit 102) based on sinking or sourcing a controlled common modecurrent through feedback resistors within the instrumentation amplifiercircuit 102. Moreover, the controlled common mode current can beutilized to drive the common mode output voltage of the instrumentationamplifier circuit 102 to a target voltage value (and/or to fall within atarget voltage range). For example, the target voltage value can be halfof the supply voltage (e.g., Vdd/2).

In one example, the current control circuit 202 can dynamically generatethe common mode current based on a feedback signal (e.g., common modeoutput voltage signal) received from the instrumentation amplifiercircuit 102. Moreover, the current control circuit 202 can control thevalue of the common mode current based on the feedback signal.Alternatively, in another example, the value of the common mode currentcan be constant, for example, in applications wherein the common modeinput voltage is known and/or can be estimated in advance. In yetanother example, the current control circuit 202 can be activated onlywhen it is detected (e.g., based on the feedback signal) that theinstrumentation amplifier circuit 102 is saturated and/or can bealways-on.

According to an embodiment, the current control circuit 202 can measurethe common mode output voltage of the instrumentation amplifier circuit102 and compare the common mode output voltage to a target voltage value(and/or range). Based on the comparison, the current control circuit 202can increase or decrease the common mode current such that common modeoutput voltage is adjusted to the target voltage value (and/or is withinthe target voltage range). As an example, the current control circuit202 can include one or more controlled current sources to facilitategeneration of the common mode current.

FIG. 2B illustrates an example system 250 that facilitates adjustment ofthe common mode output voltage (e.g., to avoid saturation of theinstrumentation amplifier circuit 102) based on controlling voltage at anode between input resistors within the instrumentation amplifiercircuit 102. In one aspect, the system 250 includes a voltage controlcircuit 252 that controls the voltage at the node to modify the currentflowing through the input resistors and thus, drive the common modeoutput voltage of the instrumentation amplifier circuit 102 to a targetvoltage value (and/or to fall within a target voltage range). Forexample, the target voltage value can be half of the supply voltage(e.g., Vdd/2).

In one example, the voltage control circuit 252 can dynamically controlthe voltage across the input resistors based on a feedback signal (e.g.,common mode output voltage signal) received from the instrumentationamplifier circuit 102. For example, the voltage control circuit 252 canmeasure the common mode output voltage of the instrumentation amplifiercircuit 102 and compare the common mode output voltage to a targetvoltage value (and/or range). Based on the comparison, the voltagecontrol circuit 252 can increase or decrease the voltage at the nodesuch that common mode output voltage is adjusted to the target voltagevalue (and/or is within the target voltage range). As an example, thevoltage control circuit 252 can include a controlled voltage source.Moreover, the voltage source can be controlled based on the feedbacksignal. In another example, the value of the voltage source can beconstant, for example, in applications wherein the common mode inputvoltage is known and/or can be estimated in advance. In yet anotherexample, the voltage control circuit 252 can be activated only when itis detected (e.g., based on the feedback signal) that theinstrumentation amplifier circuit 102 is saturated and/or can bealways-on.

As an example, systems 200 and/or 250 can be implemented by employingmost any circuits and can be utilized in different applications, suchas, but not limited to audio/media systems, automation systems,robotics, etc. In one example, systems 200 and/or 250 can be utilized toaccurately amplify sensor data collected and/or measured by a sensor(e.g., optical sensor, motion sensor, temperature sensor, pressuresensor, sound sensor, etc.) and the amplified signal can be directed toan analog to digital converter to digitize the sensor signal.

FIG. 3 illustrates an example circuit 300 utilized for adjusting commonmode output voltage of an instrumentation amplifier circuit 102 byemploying two controlled current sources, according to an aspect of thesubject disclosure. In one aspect, the instrumentation amplifier circuit102 can include two single ended amplifiers Alp (302 ₁) and Aln (302 ₂),each with a resistor Rfb (304 ₁, 304 ₂) in the feedback path betweennegative input and output, and a resistor of value 2*Rin between virtualgrounds (Np, Nn) of both amplifiers (302 ₁, 302 ₂). For purpose ofsimplifying the explanations herein, the resistor of value 2*Rin isillustrated as two input resistors (306 ₁, 306 ₂) of value Rin each inseries, with an internal node Vmid. It can be appreciated that thesubject disclosure is not limited to employing two input resistors (306₁, 306 ₂) and that the instrumentation amplifier circuit 102 can includea single input resistor of value 2*Rin that is connected between Np andNn.

As an example, Alp (302 ₁) and Aln (302 ₂) can be ideal amplifiers, withno offset and/or infinite gain, such that node Np is at same voltage asVinp, and Nn at same voltage as Vinn. Further, amplifiers Alp (302 ₁)and Aln (302 ₂) can have high input impedance (and thus no inputcurrent). Accordingly, the same current Idiff flows through all theinput resistors (306 ₁, 306 ₂). This current can be determined asfollows:

$\begin{matrix}{{Idiff} = {\frac{{V({Np})} - {V({Nn})}}{2\;{Rin}} = {\frac{{Vinp} - {Vinn}}{2\;{Rin}} = \frac{VINdiff}{2\;{Rin}}}}} & (1)\end{matrix}$wherein V(Np) is the voltage at node Np, V(Nn) is the voltage at nodeNn, Vinp is the positive input voltage, Vinn is the negative inputvoltage, VINdiff is the input voltage difference, and Rin is theresistance of the input resistor (306 ₁/306 ₂). Further, the voltageVmid at the point between the two input resistors (306 ₁, 306 ₂)corresponds to the common mode input voltage (VCMIN) and can bedetermined as follows:

$\begin{matrix}{{Vmid} = {\frac{{V({Np})} + {V({Nn})}}{2} = {\frac{{Vinp} + {Vinn}}{2} = {VCMIN}}}} & (2)\end{matrix}$Prior to introducing a common mode current (Icm), the current throughall the resistors (306 ₁, 306 ₂, 304 ₁, 304 ₂) is the same and theoutput voltages Voutp and Voutn can be determined as follows:

$\begin{matrix}{{Voutp} = {{{Vinp} + {{Rfb} \cdot {Idiff}}} = {{Vinp} + {{Rfb}\frac{VINdiff}{2{Rin}}}}}} & (3) \\{{Voutn} = {{{Vinn} - {{Rfb} \cdot {Idiff}}} = {{Vinn} - {{Rfb}\frac{VINdiff}{2{Rin}}}}}} & (4)\end{matrix}$Thus, the differential output voltage can be determined as:

$\begin{matrix}\begin{matrix}{{VOUTdiff} = {{Voutp} - {Voutn}}} \\{= {{Vinp} - {Vinn} + {{Rfb}\frac{VINdiff}{Rin}}}} \\{= {{{VINdiff}\left( {1 + \frac{Rfb}{Rin}} \right)}(6)}} \\{= {{Gdiff} \cdot {{VINdiff}(7)}}}\end{matrix} & (5)\end{matrix}$

Wherein

${Gdiff} = {1 + \frac{Rfb}{Rin}}$is the differential gain.

Further, the common mode output voltage is determined as:

$\begin{matrix}{{VCMOUT} = {\frac{{Voutp} + {Voutn}}{2} = {\frac{{Vinp} + {Vinn}}{2} = {VCMIN}}}} & (8)\end{matrix}$Thus the common mode voltage is the same at the output as at the input,but the differential mode is amplified by Gdiff. Accordingly, in oneexample, if a high differential signal is expected at the output of theinstrumentation amplifier circuit 102 (absolute value of VOUTdiff closeto the supply voltage), the common mode output voltage should be equalto or approximately equal to target voltage, such as but not limited to,half the supply voltage (Vdd/2). For example, if the common mode inputvoltage is and/or is around half the supply voltage the differentialinput signal can be amplified properly. Moreover, in this examplescenario a large differential output signal (Voutp−Voutn) can begenerated by the instrumentation amplifier circuit 102, while both ofthese output voltages (Voutp, Voutn) remain between the supply voltagesVdd and Vss (e.g., with some margin in order to ensure proper operationof the output drivers of the amplifiers). Alternatively, in anotherexample, if the common mode input voltage is close to Vss or Vdd, theamplifiers Alp (302 ₁) and/or Aln (302 ₂), can operate in saturationmode and the output voltage (Voutp or Voutn) can be clamped to Vss orVdd.

In one aspect, to prevent output voltage clamping, current controlcircuit 202 can facilitate supplying a common mode current (Icm) througheach feedback resistor (304 ₁, 304 ₂) in order to shift the common modeoutput voltage with respect to the common mode input voltage. Moreover,the current control circuit 202 can include controlled current sources(308 ₁, 308 ₂) that sink or source two matched common mode currents(Icm) to or from nodes Np and Nn. Accordingly, both output voltagesVoutp and Voutn are increased or decreased by the same voltage(Rfb*Icm), such that the differential mode output voltage remainsunchanged but the common mode voltage is increased (shifted up),assuming an Icm current flow as illustrated in the FIG. 3, or decreased(shifted down), assuming Icm flows in the opposite direction. In oneexample, the common mode currents (Icm) drive the common mode outputvoltage to be centered around a specified voltage (e.g., half of supplyvoltage) and/or lie within a specified voltage range. It can be notedthat the specified voltage can include any voltage value (e.g.,predefined and/or dynamically determined) between Vdd and Vss, forexample, that prevents an output node saturated or clamped by positiveor negative supply voltage. Based on the supplied common mode current(e.g., by the controlled current sources (308 ₁, 308 ₂)), the outputvoltages can be determined as follows:

$\begin{matrix}\begin{matrix}{{Voutp} = {{Vinp} + {{Rfb}\mspace{20mu}{Idiff}} + {{Rfb}\mspace{14mu}{Icm}}}} \\{= {{Vinp} + {{Rfb}\frac{VINdiff}{2{Rin}}} + {{Rfb}\mspace{14mu}{Icm}}}}\end{matrix} & (9) \\\begin{matrix}{{Voutn} = {{Vinn} - {{Rfb}\mspace{14mu}{Idiff}} + {{Rfb}\mspace{14mu}{Icm}}}} \\{= {{Vinp} - {{Rfb}\frac{VINdiff}{2{Rin}}} + {{Rfb}\mspace{14mu}{Icm}}}}\end{matrix} & (10)\end{matrix}$

The current Idiff flows only through the input resistors (306 ₁, 306 ₂),while the current Icm+/−Idiff flows through the feedback resistors (304₁, 304 ₂). By controlling the common mode currents (Icm), the commonmode output voltage can be adjusted. As an example, the common modecurrents (Icm) can be generated permanently in order to always force thecommon mode output voltage to equal and/or approximately equal a targetvoltage value, or the common mode currents (Icm) can be generated onlywhen required in order to avoid saturation or clamping of one of theoutput nodes. For example, a sensing circuit (not shown) can be utilizedto sense when the voltage (Voutp, Voutn) at the one of the output nodesis saturated or clamped to Vdd or Vss. In one aspect, the currentsources (308 ₁, 308 ₂) can be generated in different ways, for example,but not limited to, employing MOS transistors in their saturationregions. Both current sources (308 ₁, 308 ₂) can be matched in order tomaintain a good common mode input rejection (e.g., to avoid common modevoltage variations being transformed into differential signal) and havelow noise characteristics.

It can be appreciated that the input resistors (306 ₁, 306 ₂) and thefeedback resistors (304 ₁, 304 ₂) can have suitable resistance values(Rin, Rfb) and/or ratios) depending on the application. Further,amplifiers Alp (302 ₁) and Aln (302 ₂) can have most any gain valuedepending on the application.

FIG. 4 illustrates an example circuit 400 that facilitates common modecurrent control and provides a high common mode input rejection inaccordance with an aspect of the disclosure. In one embodiment, theinstrumentation amplifier circuit 102, amplifiers (302 ₁,302 ₂),feedback resistors (304 ₁, 304 ₂), input resistors (306 ₁, 306 ₂),include functionality, as more fully described with respect tosystems/circuits 100, 200, and 300. Further, the current control circuit202 includes functionality, as more fully described with respect tosystems 100 and 200. In contrast with the embodiment in FIG. 3, systemthe current control circuit 202 in circuit 400 employs a singlecontrolled current source 402 that provides the common mode current(2Icm). As an example, the single controlled current source 402 providesgood common mode input rejection and avoids common mode voltagevariations being transformed into a differential signal.

According to an aspect, the current control circuit 202 causes a current(Icm) to flow through the input resistors (306 ₁, 306 ₂), by sinking orsourcing a current of 2*Icm from or to the node Vmid between the twoinput resistors (306 ₁, 306 ₂). The equations of Voutp and Voutn are thesame as given by equations 9 and 10 respectively. As an example, thecommon mode current (2*Icm) drives the common mode output voltage to becentered around a specified voltage (e.g., half of supply voltage)and/or lie within a specified voltage range. It can be noted that thespecified voltage can include any voltage value between Vdd and Vss, forexample, that prevents an output node saturated or clamped by positiveor negative supply voltage. Moreover, the current control circuit 202controls the current source 402 to control the voltage at node Vmid. Forexample, the voltage at node Vmid is decreased (pulled below) orincreased (pulled above) based on the common mode current. Moreover,Vmid=VCMIN−RinIcm  (11)Thus, if the current control circuit 202 determines that common modeinput voltage (VCMIN) is low (e.g., close to Vss, within a predefinedvoltage range, below a predefined voltage threshold, etc.), then thecurrent flow of 2Icm can be in the same direction as indicated in FIG. 4in order to increase the common mode output by decreasing the voltage atVmid (e.g., the voltage at Vmid can be pulled below VCMIN).Alternatively, if the current control circuit 202 determines that thecommon mode input voltage is high (e.g., close to Vdd, within apredefined voltage range, above a predefined voltage threshold, etc.),then the voltage at Vmid can be increased (e.g., the voltage at Vmid canbe pulled above VCMIN) and the direction of the current flow 2Icm isopposite to that indicated in FIG. 4.

In an example, one advantage of the circuit 400 is that the noise of thecurrent source (2*Icm) does not contribute significantly to the noise ofthe instrumentation amplifier circuit 102, since it is only common modenoise. Differential mode noise can be detected if the resistors (306 ₁and 306 ₂, or 304 ₁ and 304 ₂) are mismatched; however such noise isstrongly attenuated. In one aspect, the common mode current 2*Icm can begenerated permanently in order to always force the common mode outputvoltage to equal, and/or approximately equal, a target voltage value, orcan be generated only when required in order to avoid saturation orclamping of one of the output nodes. For example, a sensing circuit (notshown) can be utilized to detect when the voltage at the one of theoutput nodes is saturated or clamped and accordingly activate thecurrent source 402. In one aspect, the current source 402 can begenerated by employing various circuits, for example, but not limitedto, employing MOS transistors in their saturation regions.

According to an embodiment, the value of Icm employed in circuits 300and/or 400 can be determined (e.g., by the current control circuit 202)as follows: Based on averaging equations 9 and 10,

$\begin{matrix}{{VCMOUT} = {{VCMIN} + {{Rfb}\mspace{14mu}{Icm}}}} & (12) \\{{Thus},{{Icm} = \frac{{VCMOUT} - {VCMIN}}{Rfb}}} & (13)\end{matrix}$

According to another embodiment, Icm can be generated (e.g., by thecurrent control circuit 202) only in order to avoid having one of theoutput voltages going outside the supply range or at least too close toone of the supplies (e.g., less than a predefined threshold voltage(Vdrop)). Without common mode compensation, the output voltages can beequal to:

$\begin{matrix}{{Voutp} = {{VCMIN} + {\left( {1 + \frac{Rfb}{Rin}} \right)\frac{VINdiff}{2}}}} & (14) \\{{Voutn} = {{VCMIN} - {\left( {1 + \frac{Rfb}{Rin}} \right)\frac{VINdiff}{2}}}} & (15)\end{matrix}$In an example scenario, if determined (e.g., by the current controlcircuit 202) that Voutp exceeds Vdd−Vdrop without common modecompensation, then the common mode output voltage should be decreased(e.g., by the current control circuit 202) by:

$\begin{matrix}\begin{matrix}{{Vshift} = {\left\lbrack {{VCMIN} + {\left( {1 + \frac{Rfb}{Rin}} \right)\frac{{VINdiff}}{2}}} \right\rbrack - \left\lbrack {{Vdd} - {Vdrop}} \right\rbrack}} \\{= {{- {Rfb}}\mspace{14mu}{Icm}}}\end{matrix} & (16)\end{matrix}$Thus, the current Icm that is to be generated would be determined (e.g.,by the current control circuit 202) as:

$\begin{matrix}\begin{matrix}{{Icm} = {- \frac{Vshift}{Rfb}}} \\{= {- \frac{\left\lbrack {{VCMIN} + {\left( {1 + \frac{Rfb}{Rin}} \right)\frac{{VINdiff}}{2}}} \right\rbrack - \left\lbrack {{Vdd} - {Vdrop}} \right\rbrack}{Rfb}}}\end{matrix} & (17)\end{matrix}$This value of Icm (given by equation 17) ensures that Voutp≦Vdd−Vdropand Voutn=Voutp−(1+Rfb/Rin)*VINdiff. Hence, the common mode compensationis sufficient to avoid saturation of the amplifiers (302 ₁, 302 ₂). TheIcm value in equation 17 is negative because it is a sourced current.

It can be appreciated that an absolute value of VINdiff is employed inequations 16-17, such that these equations (16, 17) also hold in anotherexample scenario wherein Voutn exceeds Vdd−Vdrop. In another examplescenario, if determined (e.g., by the current control circuit 202) thatVoutn falls below Vss+Vdrop without common mode compensation, then thecommon mode output voltage can be increased (e.g., by the currentcontrol circuit 202) by:

$\begin{matrix}\begin{matrix}{{Vshift} = {\left\lbrack {{Vss} + {Vdrop}} \right\rbrack - \left\lbrack {{VCMIN} + {\left( {1 + \frac{Rfb}{Rin}} \right)\frac{{VINdiff}}{2}}} \right\rbrack}} \\{= {{Rfb}\mspace{14mu}{Icm}}}\end{matrix} & (18)\end{matrix}$Thus, the current Icm that is to be generated would be determined (e.g.,by the current control circuit 202) as:

$\begin{matrix}{{Icm} = \frac{\left\lbrack {{Vss} + {Vdrop}} \right\rbrack - \left\lbrack {{VCMIN} + {\left( {1 + \frac{Rfb}{Rin}} \right)\frac{{VINdiff}}{2}}} \right\rbrack}{Rfb}} & (19)\end{matrix}$This value of Icm (given by equation 19) ensures that Voutn≧Vss+Vdropand Voutp=Voutn+(1+Rfb/Rin)*VINdiff. It can be appreciated that anabsolute value of VINdiff is employed in equations 18-19, such thatthese equations (18, 19) also hold the case where Voutp (instead ofVoutn) would fall below Vss+Vdrop.

Referring now to FIG. 5, there illustrated is an example circuit 500that facilitates controlling voltage at a node between the inputresistors (306 ₁, 306 ₂) to modify the common mode output voltage inaccordance with an aspect of the innovation. In one embodiment, theinstrumentation amplifier circuit 102, amplifiers (302 ₁,302 ₂),feedback resistors (304 ₁, 304 ₂), input resistors (306 ₁, 306 ₂),include functionality, as more fully described with respect tosystems/circuits 100, 250, 300, and 400. Further, the voltage controlcircuit 252 includes functionality, as more fully described with respectto systems 100 and 250.

According to an aspect, voltage control circuit 252 employs a voltagesource 502 to control the voltage at node Vmid. Moreover, the commonmode output can be controlled by applying the voltage (Vcm) provided byvoltage source 502 at Vmid, since the impedance on Vmid is directlydetermined by the two input resistors (306 ₁, 306 ₂) in parallel. As anexample, the controlled current source 402 (in FIG. 4) can be replacedby the controlled voltage source 502, as illustrated in FIG. 5. It canbe appreciated that the voltage at node Vmid can be controlled by mostany voltage source (e.g., with non zero impedance), or most any currentsource (e.g., with non infinite impedance).

In one aspect, the voltage source 502 is controlled (e.g., by thevoltage control circuit 252) to drive the common mode output voltage tobe centered around a specified voltage (e.g., half of supply voltage)and/or lie within a specified voltage range. It can be noted that thespecified voltage can include any voltage value between Vdd and Vss, forexample, that prevents an output node being saturated or clamped atpositive or negative supply voltage. For example, the voltage at nodeVmid is decreased (pulled below) or increased (pulled above) to thecommon mode input voltage based on the voltage provided by the voltagesource 502. Accordingly, if the voltage control circuit 252 determinesthat the common mode input voltage (VCMIN) is low (e.g., close to Vss,within a predefined voltage range, below a predefined voltage threshold,etc.), then the common mode output voltage can be increased bydecreasing the voltage at Vmid (e.g., the voltage at Vmid can be pulledbelow VCMIN); else if the voltage control circuit 252 determines thatthe common mode input voltage is high (e.g., close to Vdd, within apredefined voltage range, below a predefined voltage threshold, etc.),then the voltage at Vmid can be increased (e.g., pulled above VCMIN) todecrease the common mode output voltage.

According to an embodiment, the voltage (Vcm) supplied by the voltagesource 502 can be determined (e.g., by the voltage control circuit 252)as follows:Vcm=VCMIN−RinIcm  (20)Based on equations 20 and 13,

$\begin{matrix}{{Vcm} = {{VCMIN} - {\frac{Rin}{Rfb}\left( {{VCMOUT} - {VCMIN}} \right)}}} & (21)\end{matrix}$

In another embodiment, voltage (Vcm) can be determined (e.g., by thevoltage control circuit 252) as a minimum voltage value that issufficient to prevent one of the output voltages to be outside thesupply range or to be too close to one of the supplies (e.g., less thanVdrop). From equations 17 and 19, Icm can be determined as:

${{Icm} = {- \frac{\left\lbrack {{VCMIN} + {\left( {1 + \frac{Rfb}{Rin}} \right)\frac{{VINdiff}}{2}}} \right\rbrack - \left\lbrack {{Vdd} - {Vdrop}} \right\rbrack}{Rfb}}};{or}$${Icm} = \frac{\left\lbrack {{Vss} + {Vdrop}} \right\rbrack - \left\lbrack {{VCMIN} + {\left( {1 + \frac{Rfb}{Rin}} \right)\frac{{VINdiff}}{2}}} \right\rbrack}{Rfb}$Accordingly, Vcm can be determined (e.g., by the voltage control circuit252) asVcm=VCMIN−Rin*Icm  (22)

FIG. 6 illustrates an example voltage control circuit 600 utilized tocontrol the common mode output voltage. In one aspect, the voltagecontrol circuit 252 includes functionality, as more fully described withrespect to systems/circuits 100, 250, and 500. The voltage controlcircuit 252 provides a feedback loop from the output of theinstrumentation amplifier circuit 102 to the input of theinstrumentation amplifier circuit 102. According to an embodiment, thevoltage control circuit 252 determines the common mode output voltage(VCMOUT) based on a resistive divider circuit comprising resistors 602 ₁and 602 ₂. As shown in FIG. 6, VCMOUT is obtained at instrumentationamplifier circuit 102. Further, the voltage control circuit 252 caninclude a comparator circuit, for example, comprising an auxiliaryoperational amplifier (OPAMP) 604, a resistor 606 and a capacitor 608.Moreover, the comparator circuit can compare the VCMOUT with a targetvoltage value (VCMOUT_TARGET). As an example, the comparison can beachieved by applying the voltage difference between VCMOUT andVCMOUT_TARGET across a resistor 606, through the auxiliary amplifier604. The current Iinteg is integrated onto the capacitor 608 tied infeedback of the auxiliary amplifier 604. Moreover,

$\begin{matrix}{{Iinteg} = \frac{\left( {{VCMOUT} - {VCMOUT\_ TARGET}} \right)}{Ri}} & (23)\end{matrix}$In one aspect, the output of the auxiliary amplifier 604 can be directlycoupled to the node Vmid between the input resistors (306 ₁, 306 ₂)within the instrumentation amplifier circuit 102 to control the commonmode input voltage.

In one example, if VCMOUT is higher than VCMOUT_TARGET, the integratingcurrent Iinteg will flow through the resistor Ri 606 and the capacitorCi 608, and accordingly increase the voltage at Vmid. Increasing thevoltage at Vmid will then decrease the common mode output voltage, untilit reaches the target value (VCMOUT_TARGET). Alternatively, if VCMOUT islower than VCMOUT_TARGET, the integrating current Iinteg will flowthrough the resistor Ri 606 and the capacitor Ci 608 in the oppositedirection to that indicated in the FIG. 6, and accordingly decrease thevoltage at Vmid. Decreasing the voltage at Vmid will then increase thecommon mode output voltage, until it reaches the target value(VCMOUT_TARGET).

It can be appreciated that the resistors (602 ₁, 602 ₂, 606) and thecapacitor 608 can have suitable resistance values (and/or ratios) andcapacitance values respectively depending on the application. Further,auxiliary amplifier Ai (604) can have most any gain value depending onthe application. In addition, it can be appreciated that the circuitillustrated in FIG. 6 is merely one example and that the subjectdisclosure is not limited to the circuit illustrated in FIG. 6.Moreover, most any circuit for determining VCMOUT and/or comparingVCMOUT with VCMOUT_TARGET can be employed.

FIGS. 7A-7B illustrate example circuits 700 and 750 that prevent outputvoltages of the instrumentation amplifier from increasing/decreasingoutside the supply voltage range, according to an aspect of thedisclosure. In one embodiment, a current control circuit 202 includesfunctionality, as more fully described with respect to systems/circuits100, 200, and 300. The current control circuit 202 provides a feedbackloop from the output of the instrumentation amplifier circuit 102 to theinput of the instrumentation amplifier circuit 102. According to anembodiment, if output voltages Voutp or Voutn increase greater thanVdd−Vdrop, transconductance amplifiers (702 ₁ and 702 ₂) can dischargecapacitor 708 ₁, and decreasing the voltage at node Vctrlp at the inputof feedback amplifier 710 ₁. The voltage at Vctrlp is copied onto oneelectrode of two matched resistors (704 ₁ and 704 ₂) (and nodes sourcep1and sourcep2), in order to generate two matched currentsIcmp=(Vdd1−Vctrlp)/Rup1. In one example, these currents can be providedby the current control circuit 202 to the virtual grounds (Np, Nn) ofthe instrumentation amplifier circuit 102, in order to reduce the commonmode output voltage in feedback (as described in detail supra withrespect to FIG. 3). In another example, these currents can also beprovided together by the control circuit 202 to the node Vmid of theinstrumentation amplifier circuit 102, in order to reduce the commonmode output voltage in feedback (as described in detail supra withrespect to FIG. 4).

According to another embodiment, if both Voutp and Voutn are less thanVdd−Vdrop, then transconductance amplifiers (702 ₁ and 702 ₂) both pullup (increase the voltage at) node Vctrlp until it reaches Vdd. In thiscase, both p-channel metal-oxide-semiconductor (PMOS) transistors 706 ₁and 706 ₂ will be blocked, so that no current will be injected, as nocompensation is required in this case. Accordingly, additional noisesource is not added to the instrumentation amplifier circuit 102.According to yet another embodiment, if one output is greater thanVdd−Vdrop and the other one is less than Vdd−Vdrop, a current can beinjected to the virtual grounds (Np, Nn), such that the amplifier (702₁/702 ₂) with the highest input has a higher drive than the one with thelowest input. Accordingly, non linear transconductance can thus beemployed.

Similar to the above, transconductance amplifiers (702 ₃ and 702 ₄),capacitor 708 ₂, matched resistors (704 ₃ and 704 ₄), n-channelmetal-oxide-semiconductor (NMOS) transistors (706 ₃ and 706 ₄), andfeedback amplifier 710 ₂ can generate a current Icm that prevents one ofthe outputs (Voutp/Voutn) of the instrumentation amplifier circuitdecreasing below Vss+Vdrop.

FIG. 7B illustrates an example circuit 750 for supplying common modecurrents. Moreover, circuit 750 provides the same functionality ascircuit 700, without employing the resistors 704 ₁, 704 ₂, 704 ₃, and704 ₄, and feedback amplifiers 710 ₁ and 710 ₂. In this example circuit750, the currents (Icm) are directly controlled by transistors 706 ₁ and706 ₂ or 706 ₃ and 706 ₄. It can be noted that the transconductanceamplifiers (702 ₁, 702 ₂, 702 ₃, and 702 ₄), capacitors (708 ₁, 708 ₂),transistors (706 ₁, 706 ₂, 706 ₃, and 706 ₄) can include functionality,as more fully described with respect to circuit 700.

It can be appreciated that the resistors (704 ₁, 704 ₂, 704 ₃, and 704₄) of system 700 and the capacitors (708 ₁, 708 ₂) of systems 700 and750 can have suitable resistance values (and/or ratios) and capacitancevalues respectively depending on the application. Further,transconductance amplifiers (702 ₁, 702 ₂, 702 ₃, and 702 ₄) of systems700 and 750 and/or feedback amplifiers (710 ₁ and 710 ₂) of system 700can have most any gain value depending on the application. Furthermore,although transistors (706 ₁, 706 ₂, 706 ₃, and 706 ₄) are depicted asmetal-oxide-semiconductor field effect transistors (MOSFETs), most anytransistors (e.g., Bipolar junction transistor) can be utilized. Inaddition, it can be appreciated that the subject disclosure is notlimited to the circuits 700 and 750 and that most any circuit fordetermining and/or controlling Icm can be employed by the currentcontrol circuit 202.

FIGS. 8A-8B illustrate example circuits 800 and 850 that determinewhether the output voltages of the instrumentation amplifier circuit 102are within a predefined voltage range. According to an aspect, thecircuits 800 and/or 850 can be employed to control the gate (gatep) ofthe PMOSs (706 ₁ and 706 ₂) and/or the gate (gaten) of the NMOSs (706 ₃and 706 ₄) in circuits 700 and/or 750, for example, instead of utilizingthe transconductance amplifiers (702 ₁, 702 ₂, 702 ₃, and 702 ₄).

In one aspect, circuit 800 employs transistors (802 ₁, 802 ₂, 802 ₃, 802₄, and 802 ₅) to directly compare the maximum of voltages Voutp andVoutn (outputs of the instrumentation amplifier circuit 102) withVdd−Vdrop. Similarly, circuit 850 employs transistors (852 ₁, 852 ₂, 852₃, 852 ₄, and 852 ₅) to directly compare the minimum of voltages Voutpand Voutn (outputs of the instrumentation amplifier circuit 102) withVss−Vdrop. As an example, the circuits 800 and/or 850 include aclassical transconductance amplifier circuit with one transistor of thedifferential pairs (804, 854) split into two transistors (802 ₄, and 802₅, 852 ₄, and 852 ₅) with gate of one transistor (802 ₄, 852 ₄) coupledto Voutp and gate of the other transistor (802 ₅, 852 ₅) coupled toVoutn. In circuit 800, the current in the right side of the differentialpair 804 is determined by the higher voltage between Voutp and Voutn,which can then be compared with Vdd−Vdrop. In this example scenario, theoutput current (lout) is proportional, or approximately proportional, to(Vdd−Vdrop)−max(Voutp, Voutn), and can be sent to the gatep node tocontrol operation of the PMOSs (706 ₁ and 706 ₂).

In circuit 850, the current in the right side of the differential pair854 is determined by the lower voltage between Voutp and Voutn.Moreover, as the differential pair 854 is realized now by PMOS, itfacilitates comparison of Vss+Vdrop with lowest voltage of Voutp andVoutn. In this example scenario, the output current (lout) isproportional, or approximately proportional, to (Vss+Vdrop)−min(Voutp,Voutn), and can be sent to the gaten node to control operation of theNMOSs (706 ₃ and 706 ₄). It can be appreciated that the circuits 800and/or 850 can be implemented within the current control circuit 202.Further, although transistors (802 ₁, 802 ₂, 802 ₃, 802 ₄, 802 ₅, 852 ₁,852 ₂, 852 ₃, 852 ₄, and 852 ₅) are depicted MOSFETs, it can beappreciated that most any transistors (e.g., Bipolar junctiontransistor) can be utilized in circuits 800 and/or 850.

Referring now to FIG. 9, there illustrated is an example graph 900 thatdepicts output voltage of an instrumentation amplifier with a rail torail input range. Moreover, graph 900 illustrates the output voltagesVoutp 904 ₁ and Voutn 906 ₁ as a function of the common mode inputvoltage (VCMIN) 902, assuming a fixed difference (e.g., due to fixeddifferential input voltage and gain). The usable common mode input range908 is the region wherein each output Voutp 904 ₁ and Voutn 906 ₁remains within the supply voltage range (Vdd−Vss) with a voltage marginVdrop (thus between Vss+Vdrop and Vdd−Vdrop) in order to achieve properoperation of the amplifiers (302 ₁, 302 ₂) (e.g., avoiding operation inthe saturation region).

In one example scenario, if Voutp 904 ₁ is greater than Vdd−Vdrop (e.g.,the region between V2 and Vdd), the VCMOUT 910 ₁ can be shifted to 910 ₂by employing the common mode adjustment component 108 (e.g., utilizingthe current control circuit 202 or the voltage control circuit 252described in detail supra with respect to systems/circuits 200-850).Accordingly, Voutp 904 ₁ will be shifted to 904 ₂, and Voutn 906 ₁ willbe shifted to 906 ₂ (as depicted by the dotted lines). Thus, theamplifiers (302 ₁, 302 ₂) will not be saturated and errors in the outputof the instrumentation amplifier can be avoided.

In another example scenario, if Voutn 906 ₁ is less than Vss+Vdrop(e.g., the region between V1 and Vss), the VCMOUT 910 ₁ can be shiftedto 910 ₃ by employing the common mode adjustment component 108 (e.g.,utilizing the current control circuit 202 or the voltage control circuit252 described in detail supra with respect to systems/circuits 200-850).Accordingly, Voutp 904 ₁ will be shifted to 904 ₃, and Voutn 906 ₁ willbe shifted to 906 ₃ (as depicted by the dotted lines). Thus, theamplifiers (302 ₁, 302 ₂) will not be saturated and errors in the outputof the instrumentation amplifier can be avoided.

FIGS. 10-11 illustrate methodologies and/or flow diagrams in accordancewith the disclosed subject matter. For simplicity of explanation, themethodologies are depicted and described as a series of acts. It is tobe understood and appreciated that the subject innovation is not limitedby the acts illustrated and/or by the order of acts, for example actscan occur in various orders and/or concurrently, and with other acts notpresented and described herein. Furthermore, not all illustrated actsmay be required to implement the methodologies in accordance with thedisclosed subject matter. In addition, the methodologies couldalternatively be represented as a series of interrelated states via astate diagram or events. Additionally, it should be further appreciatedthat the methodologies disclosed hereinafter and throughout thisspecification are capable of being stored on an article of manufactureto facilitate transporting and transferring such methodologies tocomputers. The term article of manufacture, as used herein, is intendedto encompass a computer program accessible from any computer-readabledevice or computer-readable storage/communications media.

FIG. 10 illustrates an example methodology 1000 for increasing an inputrange of an instrumentation amplifier in accordance with an aspect ofthe subject disclosure. As an example, the instrumentation amplifier canbe employed to amplify a relatively small differential input signal, forexample, received from a sensor (e.g., resistive of capacitive bridge)while providing a high input impedance. The amplified signal can beprovided to an analog to digital (A/D) converter in order to digitizethe signal from the sensor.

At 1002, input voltage signals can be received (Vinp, Vinn). At 1004, acommon mode output voltage signal (VCMOUT) can be generated, for exampleby employing the instrumentation amplifier. As an example, the commonmode output voltage signal can be equal to or almost equal to the commonmode input voltage signal. Further, at 1006, an amplified differentialsignal (Voutp, Voutn) can be generated based on Vinp and Vinn.Furthermore, Voutp and Voutn can be compared with predefined (ordynamically defined) threshold voltages. Moreover, at 1008, it can bedetermined whether Voutp or Voutn is greater than Vdd−Vdrop. Ifdetermined that Voutp or Voutn is greater than Vdd−Vdrop, at 1010, afeedback control signal (VCM) can be adjusted in order to shift down(decrease) the common mode output voltage, for example, by shifting up(increasing) the VCM signal. Further, at 1012, it can be determinedwhether Voutp or Voutn is less than Vss+Vdrop. If determined that Voutpor Voutn is less than Vss+Vdrop, at 1014, the feedback control signal(VCM) can be adjusted in order to shift up (increase) the common modeoutput voltage, for example, by shifting down (decreasing) the VCMsignal. In one example, the common mode output voltage can be controlledby modifying a current flowing through feedback resistors of theinstrumentation amplifier. As an example, the feedback control signalcan include a controlled common mode current signal that can be providedto or sourced from the feedback resistors. In another example, thefeedback control signal can include a voltage signal applied at acontrol node between input resistors of the instrumentation amplifierthat can be modified to control the common mode output voltage.Moreover, the feedback control signal can drive the common mode outputvoltage to, or approximately to, a target voltage value (e.g. Vdd/2).Accordingly, a high amplification of the input voltage signals can beperformed independent of the common mode input voltage.

FIG. 11 illustrates an example methodology 1100 for controlling commonmode output voltage of an instrumentation amplifier, according to anaspect of the subject specification. At 1102, the common mode outputvoltage (VCMOUT) can be determined from the voltage signals generated bythe instrumentation amplifier. For example, the voltage at the output ofthe instrumentation amplifier can be averaged (e.g., employing aresistive divider circuit) to determine the common mode output voltage.Further, at 1104, the common mode output voltage can be compared to atarget voltage value VCMOUT_TARGET (e.g., Vdd/2). Furthermore at 1106,the current flowing through feedback resistors of the instrumentationamplifier can be controlled based on the comparison, for example, toavoid saturating the instrumentation amplifier and/or avoid errors dueto clamping of output voltage to Vdd/Vss. For example, if VCMOUT isgreater than VCMOUT_TARGET, the common mode output voltage can bedecreased, until it reaches the target value (VCMOUT_TARGET).Alternatively, if VCMOUT is less than VCMOUT_TARGET, the common modeoutput voltage can be increased, until it reaches the target value(VCMOUT_TARGET).

What has been described above includes examples of the subjectdisclosure. It is, of course, not possible to describe every conceivablecombination of components or methodologies for purposes of describingthe subject matter, but it is to be appreciated that many furthercombinations and permutations of the subject disclosure are possible.Accordingly, the claimed subject matter is intended to embrace all suchalterations, modifications, and variations that fall within the spiritand scope of the appended claims.

In particular and in regard to the various functions performed by theabove described components, devices, circuits, loops, systems and thelike, the terms (including a reference to a “means”) used to describesuch components are intended to correspond, unless otherwise indicated,to any component which performs the specified function of the describedcomponent (e.g., a functional equivalent), even though not structurallyequivalent to the disclosed structure, which performs the function inthe herein illustrated exemplary aspects of the claimed subject matter.Further, the components and circuitry elements described above can be ofany suitable value in order to implement the embodiments of the presentinvention. For example, the resistors can be of any suitable resistance,capacitors can be of any suitable capacitance, inductors can be of anysuitable inductance, amplifiers can provide any suitable gain, voltagesources can provide any suitable voltages, current sources can provideany suitable amperage, etc.

The aforementioned systems/circuits/components/loops have been describedwith respect to interaction between several components. It can beappreciated that such systems/circuits and components can include thosecomponents or specified sub-components, some of the specified componentsor sub-components, and/or additional components, and according tovarious permutations and combinations of the foregoing. Sub-componentscan also be implemented as components communicatively coupled to othercomponents rather than included within parent components (hierarchical).Additionally, it should be noted that one or more components may becombined into a single component providing aggregate functionality ordivided into several separate sub-components, and any one or more middlelayers, such as a management layer, may be provided to communicativelycouple to such sub-components in order to provide integratedfunctionality. Any components described herein may also interact withone or more other components not specifically described herein.

In addition, while a particular feature of the subject innovation mayhave been disclosed with respect to only one of several implementations,such feature may be combined with one or more other features of theother implementations as may be desired and advantageous for any givenor particular application. Furthermore, to the extent that the terms“includes,” “including,” “has,” “contains,” variants thereof, and othersimilar words are used in either the detailed description or the claims,these terms are intended to be inclusive in a manner similar to the term“comprising” as an open transition word without precluding anyadditional or other elements.

What is claimed is:
 1. A system, comprising: an instrumentationamplifier that amplifies a differential input voltage signal to generateoutput voltage signals; and a feedback circuit that determines a commonmode output voltage of the instrumentation amplifier based on the outputvoltage signals and generates, based on a comparison of the common modeoutput voltage with a target voltage, a controlled voltage signal thatis applied at a control node between input resistors of theinstrumentation amplifier and that adjusts the common mode outputvoltage to prevent saturation of the instrumentation amplifier.
 2. Thesystem of claim 1, wherein the feedback circuit comprises: twocontrolled current sources that generate matched common mode currentsignals, wherein the matched common mode current signals flow throughfeedback resistors of the instrumentation amplifier.
 3. The system ofclaim 1, wherein the feedback circuit comprises: a controlled currentsource that generates a common mode current that flows through inputresistors of the instrumentation amplifier.
 4. The system of claim 1,wherein the feedback circuit comprises: a controlled voltage source thatadjusts a voltage at the control node.
 5. The system of claim 4, whereinthe feedback circuit comprises: a resistive divider circuit thatmeasures the common mode output voltage of the instrumentationamplifier; and an integrator circuit that modifies a voltage at thecontrol node based on the comparison.
 6. The system of claim 5, whereinthe target voltage value is half of a supply voltage associated with theinstrumentation amplifier.
 7. The system of claim 1, wherein thefeedback circuit controls the common mode output voltage in response toa determination that at least one of the output voltage signals isclamped at a supply rail voltage.
 8. The system of claim 1, wherein thefeedback circuit increases the common mode output voltage to ensure thatthe output voltage signals are within a predefined voltage range.
 9. Thesystem of claim 1, wherein the feedback circuit decreases the commonmode output voltage to ensure that the output voltage signals are withina predefined voltage range.
 10. A method comprising: measuring outputvoltage signals from an instrumentation amplifier; determining a commonmode output voltage based on the output voltage signals; and based onanalyzing the output voltage signals, generating a control signal foradjusting the common mode output voltage to prevent saturation of theinstrumentation amplifier, wherein the generating comprises generating acontrolled voltage signal that is applied at a control node betweeninput resistors of the instrumentation amplifier based on comparing thecommon mode output voltage with a target voltage.
 11. The method ofclaim 10, wherein the generating comprises generating two controlledcurrent signals that are to be provided to a set of feedback resistorsof the instrumentation amplifier.
 12. The method of claim 10, whereinthe generating comprises generating a controlled current signal that isto be provided to the input resistors of the instrumentation amplifier.13. The method of claim 10, wherein the adjusting comprises decreasingthe common mode output voltage by a minimum voltage value that issufficient to avoid the saturation of the instrumentation amplifier. 14.The method of claim 10, wherein the analyzing comprises: comparing theoutput voltage signals with a predefined voltage range; and based on thecomparing, determining that at least one of the output voltage signalsdoes not lie within the predefined voltage range, wherein the controlsignal is generated in response to the determining.
 15. The method ofclaim 10, wherein the adjusting comprises increasing common mode outputvoltage by a minimum voltage value that is sufficient to avoid thesaturation of the instrumentation amplifier.
 16. A system, comprising: afirst circuit configured to determine a common mode output voltage of aninstrumentation amplifier based on the output voltage signals of theinstrumentation amplifier; and a second circuit configured to generate acontrol signal to minimize a difference between the common mode outputvoltage and a target voltage, wherein the generating comprisesgenerating a controlled voltage signal that is applied at a control nodebetween input resistors of the instrumentation amplifier based oncomparing the common mode output voltage with the target voltage. 17.The system of claim 16, wherein the second circuit is further configuredto determine that the common mode output voltage is greater than thetarget voltage.
 18. The system of claim 17, wherein the second circuitis further configured to increase the control signal to pull up avoltage at the control node.
 19. The system of claim 16, wherein thesecond circuit is further configured to determine that the common modeoutput voltage is less than the target voltage.
 20. The system of claim19, wherein the second circuit is further configured to decrease thecontrol signal to pull down a voltage at the control node.